Ill Bethisad is a collaborative alternative history and alternative culture universe. Its history stretches back to the mid 1990s when Andrew Smith set out to devise a conlang that would plausibly answer the question what would a modern British Romance language look and sound like? Akin to French, which is Vulgar Latin on a Gaulish substrate; he ended up with Brithenig, Vulgar Latin on a Brythonic substrate. In short order, Brithenig was given some speakers and a country to be spoken in. In late 1997, several Interested Persons began discussing the language with Andrew. Within a few years, the language itself was largely settled and other people began to take an interest in the wider world. As of last count, some threesocre people have left their mark on this shared world.
While the project began as a conlanger's collaboration, and indeed the present iteration of the project is home of many conlangs, the languages themselves no longer play such a central role. Cultural and historical description is now the order of the day.
See IB's homepage for more!